According to the CDC, there are over 20,000 deaths and 200,000 hospitalizations due to influenza each year, 90% of which were in people over age 65. The administration of the seasonal trivalent inactivated influenza vaccine remains the primary public health measure recommended by the CDC; however, this vaccine has significantly reduced efficacy, often below 50%, in this older population that needs protection most. Thus, a better understanding of immunologic and vaccination failure is urgently needed to develop better strategies to reduce influenza-related morbidity and mortality in older adults. At the tissue level, the underlying mechanisms of vaccine failure--immunologic non-responsiveness to influenza vaccination--in older adults remain to be defined. Most key immunologic responses to vaccination occur within regional draining lymph nodes (LN), not the peripheral blood. In light of this, the central novelty and innovation of this proposal is to defin the underlying B and T cell properties within the regional draining LN that can account for immunologic non- responsiveness and vaccination failure in older adults. Here we propose direct examination of influenza vaccine-induced CD4+ T and B cells within proximal LN biopsies obtained after vaccination. Recent studies in mice have identified T follicular helper (Tfh) cells key role in the development of the germinal center in the LN after vaccination to provide optimal stimulation of B lymphocytes for the development of long lasting humoral immunity with high affinity antibodies. Our preliminary data suggest that Tfh cells in blood decline with aging, but more-so for the subset of older adults who fail to respond to influenza vaccine. As such, we hypothesize that defects in T follicular helper cell (Tfh) and T helper type 1 (Th1) transcription programming, levels, phenotype, and function result in reduced antibody quantity and quality after influenza vaccination in older adults, clinically translating to non- responsiveness and vaccine failure. Moreover, we hypothesize that these defects will be most pronounced in older vaccine non-responders and contribute to the immunosenescent phenotype observed in older persons. We will address these hypotheses in the following Specific Aims: Aim 1: To determine if baseline differences in total and flu-specific CD4+ Th1/Tfh subsets and B cells in LN and blood prior to vaccination predict old influenza vaccine failures/non-responders (NR) and old vaccine-responders (R). Aim 2: To determine the specific defects in the induction of CD4+ Th1/Tfh cell and B cell responses in regional LN and blood acutely after influenza vaccination that result in the old influenza vaccine NR and R phenotypes.